You Deserve It! (Boolean Logic Operator ||)


#1

Hi beautiful code genius'. Would someone please explain why the add method within the cashRegister object utilizes the || boolean logic operator? I'm not sure what the purpose of this is.


This part:

 this.total += (itemCost || 0);


My code should work, but here it is anyway:

function StaffMember(name,discountPercent){
    this.name = name;
    this.discountPercent = discountPercent;
}

var sally = new StaffMember("Sally",5);
var bob = new StaffMember("Bob",10);

// Create yourself again as 'me' with a staff discount of 20%
var me = new StaffMember("Chris",20);

var cashRegister = {
    total:0,
    lastTransactionAmount: 0,
    add: function(itemCost){
        this.total += (itemCost || 0);
        this.lastTransactionAmount = itemCost;
    },
    scan: function(item,quantity){
        switch (item){
        case "eggs": this.add(0.98 * quantity); break;
        case "milk": this.add(1.23 * quantity); break;
        case "magazine": this.add(4.99 * quantity); break;
        case "chocolate": this.add(0.45 * quantity); break;
        }
        return true;
    },
    voidLastTransaction : function(){
        this.total -= this.lastTransactionAmount;
        this.lastTransactionAmount = 0;
    },
    // Create a new method applyStaffDiscount here
    applyStaffDiscount: function(employee){
        this.total -= this.total*(employee.discountPercent/100);
    }
    
};

cashRegister.scan('eggs',1);
cashRegister.scan('milk',1);
cashRegister.scan('magazine',3);
// Apply your staff discount by passing the 'me' object 
// to applyStaffDiscount
cashRegister.applyStaffDiscount(me);

// Show the total bill
console.log('Your bill is '+cashRegister.total.toFixed(2));
//console.log('Your bill is '+cashRegister.total)


#2

@systempro63965,
Here the explanation
https://discuss.codecademy.com/t/building-a-cash-register-7-7/27728/2?u=leonhard.wettengmx.n

If Javascript expects a Boolean true or false
and it is presented something else
it will interpret it to a truthy or falsy Value
https://developer.mozilla.org/en-US/docs/Glossary/Falsy
https://developer.mozilla.org/en-US/docs/Glossary/Truthy
Thus if you call the cashRegister.scan() Method
like
cashRegister.scan("eggs");
that would result in a
this.add(0.98 * undefined); ==> this.add( undefined );
therefore itemCost is undefined

Now if we read
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Logical_Operators
for the OR explanation

Logical OR (||) 
expr1 || expr2 
1.Returns expr1 if it can be converted to true; otherwise, returns expr2. 
Thus, when used with Boolean values, 
2.|| returns true if either operand can be converted to true; 
3.if both can be converted to false, returns false.

In our case ( undefined || 0 ) would =return= 0


#3

Makes sense! Thanks a lot for taking the time to write such a comprehensive answers. You are cool!


#4

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